Nuclear Instruments and Methods in Physics Research A 332 (1993) 506-510North-Holland

KFUPM fast neutron activation analysis facilityA. Aksoy, A.A . Naqvi, F.Z . Khiari, M . Raashid, A. Coban, R.E . Abdel-Aal and H. Al-JuwairEnergy Research Laboratory, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

Received 9 November 1992 and in revised form 11 January 1993

A newly established Fast Neutron Activation Analysis facility at the Energy Research Laboratory is described. The facilitymainly consists of a fast neutron irradiation station and a gamma ray counting station . Both stations are connected by a fastpneumatic sample transfer system which transports the sample from the irradiation station to the counting station in a short time of3 s . The fast neutron activation analysis facility has been tested by measuring the Z7AI(n, a)24Na and "5In(n, n')ti5,In crosssections at 14 .8 and 2.5 MeV neutron energies, respectively . Within the experimental uncertainties, the measured cross sections forthese elements agree with the published values .

1. Introduction

The Fast Neutron Activation Analysis technique iswidely used in the elemental analysis of geological,biological and metal samples [1] . Particularly, the tech-nique is used to determine the concentrations of shortlived radioisotopes in the samples [2] . This requires totransport the sample from the irradiation station to thecounting station in a time interval shorter than thehalf-life of the radioisotope . One of the importantapplications of the fast neutron activation technique isin the determination of oxygen contents in oil, coal andmetallic samples [3] . A Fast Neutron Activation Analy-sis (FNAA) facility has been established to carry outelemental analysis and cross section measurements atthe Energy Research Laboratory . The FNAA facilitywas tested by measuring the Z~AI(n, ot)Z4Na and115ln(n, n')ttsm In cross sections at 14 .8 and 2.5 MeVneutron energies, respectively . In the following, theKFUPM FNAA facility and the results of the tests aredescribed.

2. KFUPM fast neutron activation analysis facility

The FNAA facility mainly consists of a fast neutronirradiation station and a gamma ray counting station .Both stations are connected by a pneumatic sampletransfer system (rabbit system) with a transfer time of 3s. In the following the three main features of thefacility namely irradiation station, sample transfer sys-

i Present Address: Research Center, Saudi Arab AmericanOil Company, Dhahran, Saudi Arabia .

0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V . All rights reserved

2.1 . Fast neutron irradiation station

NUCLEARINSTRUMENTS&METHODSIN PHYSICSRESEARCH

Section A

tem and the gamma ray counting station will be de-scribed.

The fast neutron irradiation station is built aroundthe zero degree beam line of the KFUPM 350 kVaccelerator [4] . The zero degree beam line can beoperated in conjunction with either a high currentduoplasmatron ion source model 740 or a low currentduoplasmatron ion source model 820. The 740 sourcecan deliver do beams with a maximum current of 20mA. During these tests, the 820 source was used whichcan deliver few mA currents . For sample irradiations,14 .8 MeV and 2.5 MeV neutrons are produced usingT(d, n) and D(d, n) reactions, respectively . Tritium anddeuterium targets consist of tritiated/ deuteriatedatoms absorbed in titanium layers which are evapo-rated on a 0.25 mm thick copper backing. The copperbacking is cooled by a 5 mm thick water layer . A waterflow rate of 5 I/min keeps the target temperaturebelow 25°C for a 700 mA deuteron beam with 150 keVenergy . The targets were supplied by Amersham Inter-national Company, England. The target assembly ispumped by a turbomolecular pumping unit whose ex-haust is connected to the ERL tritium clean up system[5] .

2.1 .1 . Neutron flux monitoring systemIn elemental analysis using neutron activation tech-

nique, one needs to irradiate a sample and a standardto the same integrated neutron flux. This requires amonitoring of neutron flux during the irradiation of thesample and the standard . On the other hand, the flux

monitoring is also required to correct for the decay ofactivated nuclei during the irradiation period [6] . Forshort lived nuclides, the correction can be very signifi-cant depending upon the intensity and duration of theflux fluctuations . At the FNAA facility neutron flux ismonitored by a 125 mm diameter NE213 detectorplaced at an angle of 45 degrees and at a distance of 2m from the target . The NE213 detector has beenchosen due to its excellent pulse shape discrimination(psd) property. The pulse height and timing signalsfrom the detector are acquired . The psd spectrum isgenerated using a standard zero crossing pulse shapediscriminator [7] . The neutron peak of the psd spec-trum was used to gate the pulse height signal whichwas processed by a CAMAC based VAX 11/185 dataacquisition system [8,9] . In order to monitor the fluctu-ations in the neutron flux, the psd gate is also countedby a CAMAC sealer for a preset time and is stored inthe memory . Thereby it generates a neutron flux distri-bution as a function of time . Depending upon thehalf-life of the sample, the minimum preset time of theCAMAC sealer can be set to 0.2 s .

2.2. Sample transfer system

The rabbit system is a 60 m long transfer lineconnecting the irradiation station and the countingstation.The pneumatic type rabbit system was built by

A . Aksoy et al. / KFUPMfast neutron acticuton analysis facility

Two Way Divertsrs

Corridor

Fig. 1 . Layout of the FNAA facility at the ERL.

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the Nuclear Division of International Technologies ser-vices Inc. (Intertech), Florida, USA. It transports acylindrical high density polyethylene capsule with 25mm out diameter and 62 mm height. Inside the capsulethe sample is contained in a polyethylene vial with 7 mlvolume . It utilizes a pressure of 10 psi to transfer thesample and standard between the irradiation andcounting stations in a short period of 3 s. Such a shorttransfer time has allowed us to count the inducedactivity in samples with half-lifes greater than 5 s. Fig.1 shows the layout of the FNAA facility at the ERL.The sample and the standard, which are irradiated sideby side are transferred one after the other to thecounting station using two diverter units installed inthe rabbit system . Later on the used sample could bestored in a radioactive disposal box.

The operation of the rabbit system during the irra-diation and counting processes is controlled by an IBMcompatible computer. In order to control start andstop of the irradiation process, the rabbit system com-puter controls the operation of the zero degree beamline gate valve before the tritium target . This is achievedby incorporating an interface between the rabbit com-puter and the 350 keV accelerator control panel. Thecontrol program requires irradiation, delay and count-ing times as well as number of cycles as input data.Once the input data is loaded in the computer thethree processes are carried out sequentially and cycli-

erator Hall

-tik

50 8

cally. At the beginning of a cycle the sample is loadedand the rabbit system transfers it to the irradiationstation . Then the beam line gate valve is opened andthe computer starts its count down for the irradiationprocess. As soon as the irradiation period ends, therabbit computer signal closes the beam line gate valveand the sample is transfered back to the countingstation . After the elapse of the preset delay time, therabbit computer allows the data acquisition computerto count the activity for a preselected counting time .As soon as the counting period is over, the sample istransfered back to the irradiation station and the cyclerepeats itself for a given number of cycles .

Fig. 2 .

A . Aksoy et al. / KFUPMfast neutron actwition analysis facility

2 .3 . Gamma ray counting station

The counting station mainly consists of two 5 x 5 in .NaI(Tl) detectors and a 135 cm3 volume HPGe detec-tor along with a PC based data acquisition system. Thehyperpure germanium detector has a resolution of 1 .85keV for 1 .332 MeV -y-rays from "'Co . The full-energypeak efficiency of the HPGe detector was measured ina close geometry of 1 cm and far geometry of 19 cmusing standard -y-ray sources [10] . In the far geometrythe maximum efficiency of the HPGe detector wasmeasured to be 0.03-0.5% for 40-1500 keV y-rayenergy, while in the close geometry the measured effi-

PC Based Data AcquisitionSystem

Block diagram of signal processing electronics required for counting short lived induced activity in oil sample via160(n, P) 16N nuclear reaction .

ciency varied from 0.5% to 19% over the same energyrange. The energy resolution of Nal detector was mea-sured to be 8% for 662 keV -y-rays from 137CS source .The measured efficiency of the Nal detector in the fargeometry was 0.4-3.1% over the same energy range[11] . In order to increase the detection efficiency, thetwo Nal detectors can also be operated in pair, face toface . This type of application is important to count forlow level induced activity in oil samples [12] .

2.3.1 . Electronics and data acquisition systemThe counting station has signal processing electron-

ics and a PC based data acquisition system. Fig. 2shows a block diagram of the electronics required forcounting the induced activity of a sample using a pairof NaI(Tl) detectors . Also shown in the figure is theelectronics used for the neutron flux monitor. Thepulse height signals from both detectors are amplifiedin separate amplifiers and their gains are adjusted insuch a way that the full energy peak is in the samechannel of the multichannel analyzer. Then, the bipo-lar output of each amplifier is used to generate a gatethrough a single channel analyzer for a preselectedenergy range. The unipolar output of the two ampli-fiers are added in a summing amplifier which is beingused as an OR module. Also, the SCA outputs of thedetectors are mixed in a logical OR module whoseoutput is used to gate the output of the summingamplifier. An IBM compatible 80386 PC is convertedinto a multichannel analyzer using a Multiplexer model476 and an ADCAM multichannel buffer with 8kmemory model 918A supplied by EG &G-ORTEC,USA. A home made gate mixer unit shown in fig . 2gates the multichannel buffer during irradiation andcounting of sample and standard. The data acquisitionprogram MAESTRO supplied by EG&G-ORTEC isused to acquire the data . This program has standardfeatures like peak integration, peak fitting, smoothingetc. Apart from this, a software package OMNIGAMis installed on the PC to analyze the gamma spectrausing data libraries .

3. FNAA facility tests

Since the facility is to be used for neutron activationexperiments utilizing 14 .8 and 2.5 MeV neutrons, itstests were carried out by measuring the 27AI(n, a )24Naand 115In(n, n')"5-In cross sections at 14 .8 MeV and2.5 MeV neutron energies, respectively . A 14 .8 MeVneutron flux was produced by bombarding a 3 Ci/in.2tritium target with 19 mA current of 150 keV deuteronbeam while 2.5 MeV neutron flux was produced bybombarding a 1.2 cm3/in.2 deuterium target with 10mA current of 100 keV deuteron beam . During the

A . Aksoy et al. / KFUPMfast neutron actiuition analysis facility

4. Summary

Acknowledgement

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irradiation period, the neutron flux was monitored by a125 mm diameter NE213 detector . In both cases sam-ples with a purity of 99.0-99.9% were used .

In 2 AI(n, U)24Na cross section measurement Al foilsamples with 1 mm thickness, were irradiated for 4.5 hutilizing a flux of about 10 6 neutrons/ cm2/s of 14 .8MeV neutrons . The induced activity in the Al samplewas counted via 1.368 MeV -y-line of 24Na using theHPGe detector [10] . In order to identify the selected-y-line, the half-life of 24Na was determined from theslope of the activity as a function of time . The mea-sured value of the half-life 14.96 + 0.37 h agrees withthe published value of 15.02 h [13] . The cross section of27AI(n, a)24 Na measured in this experiment was 97 + 6mb which agrees with the published value of 111 + 4mb [14]. For the facility test with 2.5 MeV neutrons,indium foils samples with 0.1 mm thickness, were irra-diated for 4.5 h utilizing a flux of about 105neutrons/cm2/s of 2.5 MeV neutrons. The inducedactivity in the Indium sample was counted using 0.336MeV -y-line of 115m In with the HPGe detector. Themeasured value of the half-life of the induced activitywas 4.39 + 0.11 h which agrees with the publishedvalue of 4.5 h [13]. The cross section of 115In(n, n')115mIn measured in this experiment was 204+ 9mb which agrees with the value of 188 + 11 mb pub-lished in the literature [15] . These tests have shownthat the newly established Fast Neutron ActivationAnalysis Facility at King Fahd University of Petroleumand Minerals is capable of measuring the fast neutronactivation cross sections with a reasonably good accu-racy.

The FNAA facility at Energy Research Laboratoryhas been established . It mainly consists of a fast neu-tron irradiation station and a gamma ray countingstation . Both stations are connected by a fast sampletransfer system with a transfer time of 3 s. The facilitywas tested by measuring the known "AI(n, a)'Na and"5In(n, n')"In cross sections at 14 .8 and 2.5 MeVneutrons, respectively . Within the experimental uncer-tainties, the measured cross sections agree with thepublished values in the literature .

This work is part of Energy Research LaboratoryProject supported by the Research Institute of KingFahd University of Petroleum and Minerals, Dhahran,Saudi Arabia.

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A . Aksoy et al. / KFUPMfast neutron acttuition analysis facility

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